(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to a normally-white twisted-nematic-mode (TN-mode) LCD device including liquid crystal (LC) molecules having a twisted angle of around 90 degrees.
(b) Description of the Related Art
In general, TN-mode LC devices include first polarization film, first glass substrate, LC layer, second glass substrate, and second polarization film, which are arranged in this order as viewed from the light incident side thereof. The LC layer includes therein LC molecules having a longer axis aligned parallel to the substrate surface upon applying no electric field thereto, and the LC molecules are twisted by 90 degrees in the longer axis thereof from the first substrate to the second substrate. In the normally-white TN-mode LCD device, the first and second polarization films are arranged so that the polarization axes thereof are disposed perpendicular to each other and thus the LCD device exhibits white color upon application of no electric field.
There are two modes of the TN-mode LCD device as to the basic relationship between the arrangement of the polarization axis of the polarizing films and the orientation of the LC molecules: an e-mode wherein the transmission axis of a polarization film is parallel to the longer axis of the LC molecules on the interface between the LC layer and the substrate near the polarization film; and an o-mode wherein the transmission axis of a polarization film is perpendicular to the longer axis of the LC molecules on the interface between the LC layer and the substrate near the polarization film.
A viewing angle characteristic, showing a range of the viewing angle in which the LCD device achieves a specific contrast ratio or above, is known as one of the important indexes of the performances of the LCD device. The specific contrast ratio employed heretofore is 10:1, for example, as a luminance ratio of white color to black color measured in a printed sheet wherein a high-quality sheet of white paper is printed with a black ink. In a catalogue of the LCD devices, for example, the viewing angle characteristic of the vertical direction and the horizontal direction is listed for each LCD device, with the vertical direction of the LCD device being set parallel to the longer axis of the LC molecules residing at the middle of the LC layer between the substrates upon display of white color, and with the horizontal direction being set parallel to the longer axis.
It is known in the TN-mode LCD device that the refractive index anisotropy of the LC layer reduces the contrast ratio in a slanted viewing direction to degrade the viewing angle characteristic of the LCD device. Patent publication No. JP-A-5(1993)-19249 describes a solution for the problem of the refractive index anisotropy by using an optical compensation film. FIG. 7 shows the described LCD device in an exploded view. The LCD device 200 includes an LC cell 10 having therein an LC layer sandwiched between a pair of glass substrates, a retardation film 13, 14 between a light-incident-side polarization film (referred to as polarizer hereinafter) 11 and the LC cell 10 as well as between a light-emitting-side polarization film (referred to as analyzer hereinafter) 12 and the LC cell 10.
The transmission axes P1, P2 of the polarizer 11 and analyzer 12 are perpendicular to each other. The LC layer in the LC cell 10 has an orientation A0 parallel to the transmission axis P1 of the polarizer 11 in the vicinity of polarizer 11. The LC layer also has an orientation B parallel to the transmission axis P2 of the analyzer 12 in the vicinity of the analyzer 12. In addition, the light-incident-side orientation A0 of the LC layer is substantially parallel to the slow axis R1 of the retardation film 13, whereas the light-emitting-side orientation B of the LC layer is substantially parallel to the slow axis R2 of the retardation film 14. It is recited in the patent publication that setting of the relationship between the orientation of the LC layer and the retardation as such suppresses reduction in the contrast ratio in a slanted viewing direction, to thereby improve the viewing angle characteristic of the LCD device.